In a general aspect, the present invention relates to a cable to be used to transmit current in condition of so-called superconductivity, i.e. in conditions of almost null electric resistance. More particularly, the invention relates to a superconducting cable for high power having at least one phase, including a superconducting core comprising a phase conductor and a neutral conductor, external to the former and coaxial to the same, each including at least a layer of superconducting material, said coaxial conductors being electrically insulated from one another by interposition of a dielectric material, as well as means for cooling said core at a temperature not higher than the critical temperature of said superconducting material.
In the following description and the subsequent claims, the term: xe2x80x9ccable for high powerxe2x80x9d, indicates a cable to be used for transmitting current quantities generally exceeding 5,000 A, such that the induced magnetic field starts to reduce the value of the maximum current density achievable in superconductivity conditions.
In the following description and the subsequent claims, the termxe2x80x9d superconducting material, indicates a material, such as for instance special niobium-titanium alloys or ceramics based on mixed oxides of copper, barium and yttrium, or of bismuth, lead, strontium, calcium, copper, thallium mercury, comprising a superconducting phase having a substantially null resistively under a given temperature, defined as critical temperature of Tc.
The term: superconducting conductor, or shortly, conductor, indicates in the following any element capable of transmitting electric current in superconductivity condition, such as for instance a layer of superconducting material supported by a tubular core, or tapes of superconducting material wound on supporting core.
As is known, in the field of energy transmission, one of the problems more difficult to solve is that of increasing as much as possible both the current to be transmitted in superconductivity conditions and the temperature at which the transmission takes place,
Even though the so-called xe2x80x9chigh-temperaturexe2x80x9d superconducting materials are available today, which can transmit currentings at temperatures of the order of 70-77?K. (about xe2x88x92203/xe2x88x92196 BC), a reduction in the current transmission capacity by said material is noticed when the inducted magnetic field increases.
See on the matter, for instance, T. Nakahara xe2x80x9cReview of Japanese RandD on Superconductivityxe2x80x9d, Sumitomo Electric Technical Review, Nr. 35, January 1993.
In superconductivity conditions, the sensitivity of superconducting materials to the effects of the inducted magnetic field is ever more marked the greater is the working temperature of the superconducting core of the cable (i.e., the superconducting materials with the highest critical temperature are more sensitive to the effects of the magnetic field), so that in practice high-temperature superconducting materials do not allow to transmission current higher than some KA, on pain of an unacceptable increase in the quantity of superconducting materials to be used, and, along therewith, of the associated costs.
In the case of the so-called coaxial cables, whose configuration is suitable to transmission high loads, the induced magnetic field, the transmitted current and the diameter of the conductor are tied by the following relation:
B=(xcexcoxc3x97I)/(xcfx80xc3x97D)
wherein:
B=magnetic field on the surface of the conductor;
I=transmitted current;
xcexco=magnetic permeability;
D=diameter of the conductor.
(As is known, the values of B and I are to be understood as direct current actual values, or as alternate current effective values).
On the basis of this relation, it ensues that each increase in the transmitted current brings about a proportional increase in the induced magnetic field, which in turn limits, to a greater or smaller extent, the maximum current density obtainable in superconductivity conditions or technical critical current density, xe2x80x9cJexe2x80x9d, defined as the ratio between the critical current and the total cross section of the layer of superconducting material.
More particularly, it has been noticed that the critical current density drastically decreasesxe2x80x94sometimes up to two orders of magnitudexe2x80x94starting from a threshold value of the magnetic field, lower than the critical field above which the superconductivity is substantially compromised; indicatively, such value varies from 0.1 to 20 mT according to the superconducting material used and to the working temperature; in this regard, reference is made to, for instance, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 5, nr. 2, June 1995, pp. 949-952.
The attempts made to keep the critical current density at acceptable values based on an increase in the conductor diameter, have till now failed, due to both the practical difficulty of making, transporting and installing a large diameter cable, and the high costs necessary to cool the superconducting core, being the thermal dissipations proportional to the diameter of the insulating layer that surrounds the core of the superconductor.
Therefore, in view of these difficulties of technological nature, in the field of coaxial cables the art has been substantially restricted to either transmitting the desired high current quantities by means of suitable metal or ceramic materials at the temperature of 41K, at which the aforementioned phenomena are less marked, or accepting an other than optimum exploitation of the superconducting material at the maximum temperature (650-900K) compatible with current transmission in superconductivity conditions.
In the first case, one has to face the high costs associated with the need of cooling the superconducting core at a very low temperature, while in the second case it is necessary to use a very high quantity of superconducting material.
According to the invention, it has now been found that the problem of transmitting within a coaxial cable having at least one phase high current quantities at the maximum working temperature of the superconducting materials available today (650-900K, determined by the usable materials and cooling fluids) can be solved by splitting up for each phase the superconducting material within the cable into a plurality of xe2x80x9cnxe2x80x9d elements, structurally independent and magnetically uncoupled, each of which comprises a couple of phase and neutral coaxial conductors, insulated from one another, and transmits a fraction xe2x80x9cI/nxe2x80x9d of the total current.
According to the invention, in fact, it has been found that with such distribution of the superconducting material it is possible to:
a) reduce the size of the cable, with the same use conditions of the superconducting material, with the ensuing easiness of construction, transport and installation of the cable;
b) use, with the same quantity of superconducting material, the same quantity of electric insulating material of conventional cables;
c) limit, with the same quantity of superconducting material, the size of the thermal insulation layers (cryostat) which surround the superconducting core of the cable, with an advantageous reduction in thermal losses;
d) have superconducting elements which, in case of need, can independently supply different loads.
Preferably, the phase and neutral coaxial conductors of each of said elements comprise a plurality of superimposed tapes of superconducting material, wound on a tubular cylindrical support, for instance made of metal or insulating material in order to reduce as much as possible the possible mechanical stresses in their inside, the tapes of superconducting material are wound on said support according to windup anglesxe2x80x94either constant or variable from tape to tape and within each individual tapexe2x80x94of from 10B and 60B.
Alternatively, the phase and neutral coaxial conductors of each of said elements may comprise a plurality of layers of superconducting material, superimposed and laid on the tubular cylindrical support.
According to the invention, the maximum number of coaxial conductive elements is determined by the minimum diameter of such elements compatible with the winding deformations of the tapes made of superconducting material, or anyhow compatible with the critical tensile deformation of the superconducting material chosen.
Preferably, the diameter of the phase conductor of each of said elements varies from 25 to 40 mm.
According to the invention, the superconducting core of the cable is cooled at temperatures not higher than 650-900K, advantageously using so-called high-temperature superconducting materials and liquid nitrogen as cooling fluid.
Among these high-temperature superconducting materials, use may advantageously be made of those known in the art by the initials BSCCO having the formula:
Bixcex1Pbxcex2Srxcex3Caxcex4Cuxcex5Ox
wherein:
xcex1 is a number of from 1.4 to 2.0; xcex2 is a number of from 0 to 0.6; xcex3 is a number of from 0 to 2.5; xcex4 is a number of from 0 to 2. 5; xcex5 is a number of from 1. 0 to 4. 0; x is the stoichiometric value corresponding to the different oxides present.
According to the invention, particularly preferred are mixed oxides of the following ideal general formula:
(BiPb)2Sr2Canxe2x88x921CunOx
wherein n is a whole number of from 1 to 3 and x is the stoichiometric value corresponding to the different oxides present.
Among them, particularly advantageous results have been obtained with the mixed oxide known as BSCCO-2223 (i.e., in which n=3), or with suitable mixtures of mixed oxides of the aforementioned metals, in such ratios as to obtain a mean stoichiometry of the mixture corresponding to that of the BSCCO-2223 oxide.
In another aspect, the present invention relates to a method for transmitting a current quantity higher than a prefixed value within a superconducting cable having at least one phase, which method is characterized in that said current is split up, for each phase, -among a plurality of magnetically uncoupled conductive elements of a coaxial type, the number of such conductive elements being such that the current fraction carried in each of them is lower than a value which determines a superficial current density corresponding to a magnetic field capable of generating a conductivity reduction of a superconducting material used. In a particular embodiment, such current is a multiphase alternate current, and said conductive elements among which the current is split up, carry a single phase of said current.
In a preferred embodiment of the method, said prefixed quantity of current is at least equal to 5,000 A. In the method according to the invention, and if liquid nitrogen is used as cooling fluid, the magnetic field capable of generating a conductivity reduction of the superconducting material used is lower than 200 mT, preferably lower than 100 mT and more preferably lower than 20 mT.